1. Field of Invention
The present invention relates to laser devices, and more particularly to ion-laser devices employing hollow cathode electrode structures.
For ion-lasers, the pertinent transitions of energy states necessary for laser action are associated with the excited states of singly or multiply ionized gas. To cause these excited states it is necessary that the density of charged particles (i.e., ions) present in the laser as well as the degree of their ionization (and hence the current density) be very high.
To achieve laser action with relatively modest currents, conventional ion-lasers generally use either a water-cooled quartz capillary, a refractory material capillary, a graphite annulus, or segmented metal rings to both constrict the discharge current and to provide the requisite high degree of ionization and excitation rate. Conventional lasers constructed in this manner suffer from serious deficiencies that make them inefficient, relatively shortlived, and unreliable.
The high input power density that is used creates a severe thermal dissipation problem which ultimately limits the maximum attainable optical output power of the device. For example, the interface between the quartz capillary and the cooling medium is of very small area; since quartz is a notoriously poor conductor of heat, a limitation is placed on the power dissipation that can be tolerated per unit length of capillary. An even more serious problem is physical change of the quartz itself -- such as erosion, sputtering, strains, and eventual destruction of the capillary walls caused by contact with the high temperature plasma and by ion bombardment in a high current density discharge. In the pressure range in which the capillary tubes are operated, the ion mean-free-path is comparable to the bore of the discharge tube. As a result, an ion can be accelerated into the capillary wall with a kinetic energy of several electron volts. Collision of these high energy ions with the walls causes the quartz to gradually decompose and darken, thereby imposing a serious limitation on the useful life of ion lasers. Gas cleanup and cathophoresis are also related serious problems.
More importantly, since the surface-to-volume ratio of the discharge column is extremely large, and since in these prior art devices the plasma is in physical contact with the wall, a high rate of both energy dissipation and ion depletion (through surface recombination) will result, thereby reducing considerably the overall efficiency of existing ion lasers.
As is well known for continuous wave ion lasers, both gain and beam power output increase rapidly with increasing discharge current density. But since the current density J of conventional ionlasers must be limited to relatively low levels because of the limited thermal dissipation capacity of the quartz capillary, these lasers are operated under conditions which do not result in favorable output efficiencies. For example, in a 4880A argon ion-laser, the output power varies exponentially with current density J as J.sup.6 at low current densities falling off to J.sup.4 at high current densities (on the order of 100 amp/cm.sup.2). At still higher current densities, both the gain and the output power display a J.sup.2 dependence. This indicates that the production of radiating ionic states, such as the laser levels, proceeds by electron excitation of ions to the optical state from the ion ground state or from the ion metastable state, or both. Therefore the density of excited ions in the upper laser state increases quadratically with electron density and thus with discharge current density.
Trapping of resonance radiation from the lower laser level to the ion ground state (reabsorption of radiation by ground state ions to bring the ion back to an excited lower level of the lasering transition which occurs at extremely high current density (on the order of 10.sup.3 amp/cm.sup.2) can destroy the population inversion necessary for laser operation and thus reduce the output power to zero.
Operation at the current density corresponding to peak gain and peak output power is not possible for prior CW ion lasers because of the serious above-described material problems associated with thermal dissipation and ion bombardment.
Other problems which arise from the requirement for the necessary plasma include maintenance of ionization of the gas and the related problem of the choice of a suitable E/p ratio for the particular laser gas. In addition, because of the high power per unit volume normally associated with high power lasers, removal of heat through walls of the discharge tube is another related problem of prior art devices. Further, in some prior art devices it is necessary to cool the laser gas itself. Also, the requirement for a high density of electrons (and for particular values of E/p) usually results in occurrence of large plasma currents which strain the electrodes.
In molecular gas lasers there is a tendency for the plasma to become unstable, particularly at higher pressures. In this event the plasma will constrict and form filaments which concentrate on limited portions of the cathode. The unstable plasma will not uniformly fill the laser area; in many cases it will reduce the output of the laser.
Another object of the instant invention is to contain a plasma region in a gas laser so that the plasma is not in contact with containing walls.
Yet another object of the present invention is to maintain an abnormal glow discharge within a hollow cathode structure by use of a coaxially-mounted auxiliary anode surrounding the cathode.